Methods are known for the production of molded articles with a noncellular skin and a cellular core from polymer materials by injecting a gas into the melt or by the use of polymer material mixed in advance with a blowing agent. In both cases, as a result of the partial foaming which begins during the filling of the mould, the obtained articles have a characteristically rough surface.
Attempts have been made to avoid this disadvantage and to obtain parts with smooth surfaces. Some of these methods use the rheological properties of polymer melts to make it possible for the material last introduced into the mold to reach the zones most remote from the sprue. In one of these methods, a smooth surface is achieved by initially introducing into the mold a plastified material which does not contain any blowing agent, and thereafter introducing a material containing a blowing agent. Other methods avoid premature foaming by initially providing a gas pressure in the mold cavity which is higher than the partial pressure of the gaseous products evolved upon the decomposition of the blowing agent. Foaming in these cases is effected by permitting part of the foamable material to expand into a cylinder or by increasing the volume of the mold cavity.
A basic disadvantage of these methods is that they are applicable in practice only for relatively thick-walled articles, since the withdrawal of the foamable material in the case of thin-walled articles is difficult to achieve; and in cases in which foaming occurs as a result of an increase of the mold volume by partial opening of the mold, the filling of the mold is also difficult because of the small wall-thickness. Large injection and clamping forces are necessary and lead to the use of bulky machines. The filling of the mold in the case of thin-walled parts is also complicated as a result of the difficulty of removing the gases from the mold cavity, as well as by the cooling effect of these gases on the flow front of the foamable material which is introduced into the mold.
The apparatus for practicing these methods generally comprise additional plastifying and injecting units, as well as suitable nozzles. The disadvantage of these devices are then limitations with regard to the shape of the molded articles, the comparatively high relative weight, and the need for the aforementioned additional plastifying and distributing units.